Pressure on government critics has been mounting since President Mahinda Rajapakse was re-elected on January 26, defeating his former army chief Sarath Fonseka, the London-based human rights group said.
"Victory against the Tamil Tigers followed by a historic election should have ended political repression in Sri Lanka, but instead we have seen a serious clampdown on freedom of expression," said Madhu Malhotra, Amnesty's Asia-Pacific Deputy Director.
Amnesty cited the post-poll arrests of opposition supporters and journalists, death threats against several prominent newspaper editors and the harassment of trade unionists.
A pro-opposition newspaper was raided, several websites supporting Fonseka were blocked while Prageeth Eknaligoda, who wrote for the Lanka e-news website, disappeared on his way home from work two days before the election.
Sri Lankan journalists have given Amnesty a list of 56 of their colleagues who face serious threats, including some attached to state-run med